The use of semi-continuous or continuous systems utilising different fluids (e.g. carbon dioxide) in their supercritical state have been extensively studied in relation to pharmaceutical and other fine particle formation. Continuous hydrothermal processes have been used to synthesize nano-scale fine particles (diameter typically <100 nm) including, but not restricted to: pure metals, metal oxides, metal chalcogenides or other ceramics, or intimate mixtures of one or more of these.
Specifically, the continuous hydrothermal process involves rapidly mixing purified water at an elevated temperature and pressure with an aqueous precursor at a lower temperature, to yield a combined stream at an intermediate temperature. Typically the resulting mixture is close to or above the critical point of the purified water (the critical temperature, Tc=374° C.; critical pressure, Pc−22.1 MPa) to ensure that the reactions are rapid. This is achieved by heating the water to a temperature above Tc at a pressure above Pc, whilst the precursor remains at close to room temperature.
Approaching the critical point, as the temperature of the purified water increases at constant pressure (e.g. 25 MPa), the solubility of a dissolved precursor decreases sharply. At the same time, the equilibrium reaction H2O=[H+]+OH″] shifts to the right with increasing temperature. Thus, as the pure water and aqueous solution are mixed, many nanoparticles are formed by rapid nucleation, owing to the formation of a highly supersaturated mixture. The formation of metal oxides in this chemical environment is thought to occur by a mechanism of hydrolysis followed by rapid dehydration, owing to the excess of [OH″] and [H+] ions. The formation of an oxide of a metal M from the complex salt MLx (where L can be a nitrate or acetate anion for example) may be written as: Hydrolysis:MLx+xOH″→M(OH)x+xL″,  (1)Dehydration: M(OH)x→MOx/2+(x/2)H2O.  (2)
The density of pure water also decreases rapidly as it is heated to above the critical point at constant pressure. Thus, significantly, the densities of the aqueous precursor may be many times higher (typically up to ten-fold) than the supercritical water. Consequently, the problem of ensuring the rapid and intimate mixing of the two streams with widely differing densities is not a trivial one, because such differences tend to inhibit mixing.
A known mixing device comprises a simple ‘tee’ shaped tubular fitting inside which directly opposing or orthogonal flows of an aqueous precursor and supercritical water are brought into contact. Ideally, the nanoparticles are continually formed at the tube junction such that it is carried away as an aqueous slurry via the third branch of the ‘tee’. However, owing to complex flow patterns arising from differences in density (i.e. buoyancy driven flow) it is not easy to control the precise location where precipitation occurs in such an arrangement, and moreover the location may not be stable as the reaction proceeds. Accordingly, it is not uncommon for undesirable blockages to occur in the tubular fitting, prohibiting the running of the apparatus for an extended period. EP-A-1713569 describes the limitations of ‘Tee’ and ‘Y’ shaped reactors.
Blockages are a considerable hindrance to the manufacture of nanoparticles, and in fact the resulting obstruction of the flow is highly dangerous due to the very high mixer pressures of ca. 25 MPa.
EP-A-1713569 proposes a solution to the blockage of reactors, and provides a counter-current mixing reactor whereby opposed streams of precursor and supercritical water are brought together, and the outflow of suspended nanoparticles is around one of the inlets to the reactor; a heater is provided around the outlet. This arrangement is said to avoid pre-mixing or stagnation, thus, minimizing blockage of the reactor or the pipework associated therewith.
Continuous hydrothermal systems are currently used to synthesise a variety of nanomaterials, however, several materials have emerged as being improved by synthesis in continuous hydrothermal systems. Nano-sized ZnO is an example of a material that has received a lot of attention in the continuous hydrothermal literature and has been synthesised using a variety of reaction point geometries, precursors and reaction conditions. The reaction mechanisms governing the formation of ZnO in continuous systems are well understood. The effects of processing parameters are also known in conjunction with the crystallinity, morphology and yield of the material. Nanosized ZnO is useful in many applications such as sunscreens, paints, varnishes, plastics, cosmetics and broad UV-A and UV-B attenuation agents.
Hydroxyaparite (HA) is an example of a solid non-metal nano-material synthesised using continuous hydrothermal methods. The crystallization through rapid heating of a co-precipitate, formed by mixing of calcium nitrate and diammonium hydrogen phosphate in a basic environment, is thought to yield a nano-sized product (in at least one dimension. Hydroxyaparite is used in many applications, inert biological coatings and hard tissue replacements being amongst the most prevalent.